Recently, a Cu wiring having lower electric resistance than an Al wiring is used as a wiring of LSI. On the other hand, an insulating film called a low-k film having lower dielectric constant than a silicon oxide film is used as an inter-wiring insulting film.
Cu has a diffusion coefficient larger than Al, and then it can not be used as wiring material so long as an anti-diffusion film is employed. A conductive anti-diffusion film such as a Ta film is used with respect to side and bottom surfaces of the Cu wiring.
On the other hand, an insulative anti-diffusion film such as a silicon nitride film has been conventionally used with respect to an upper surface of the Cu wiring.
However, adhesion between the Cu wiring and the insulative anti-diffusion film is weak, and then electromigration (EM) lifetime has notably lowered along with a scaling of wiring. Therefore, a metal cap is used for the purpose of improving EM lifetime (see JP 2007-157959, JP 2007-103850).
The metal cap is formed by using plating process. The reason employing the plating process is that it makes possible to form the metal cap selectively on the upper surface of the Cu wiring.
However, the conventional method for forming the metal cap employing the plating process fails in plating selectivity, and then metal material of the metal cap is also deposited on the upper surface of the insulating film between the wirings. This deposited metal material (metal residue) causes a leakage path, and then leakage current between the wirings increases when a certain voltage higher than predetermined level is applied between the wirings.